1. Field of the Invention
The present invention relates to a device for measuring, at a plurality of points, the microwave field diffracted by an object illuminated by a microwave radiation source at a frequency and comprising:
at each of said points an electric doublet type antenna loaded at its center by a diode,
means for collecting the diffracted microwave radiation and delivering a collected microwave signal,
means for generating a low frequency signal,
multiplexing means, disposed between said generating means and each of said diodes, and
means for controlling said multiplexing means so that at least one of said diodes is biassed by said low frequency signal and, in response to said low frequency signal and said collected microwave signal, for generating a signal representative of the microwave field at the point where said doublet antenna loaded by said biassed diode is located.
Such a device may be used for numerous industrial applications and in particular for the non destructive control of objects likely to present certain defects, or else materials or products having undergone transformations such as drying, de-freezing or polymerization, for example, so as to check the results of such transformation.
2. Description of Related Art
In fact, it is known that a body illuminated by a microwave radiation source diffracts this radiation in a way which obviously depends on its shape, but also on the dielectric constant and on the conductivity at every point of its volume. Since these last two quantities depend obviously on the internal structure of the body, it is therefore possible to carry out non destructive control of the internal structure of a body by analyzing the diffraction of a microwave field illuminating this body.
By diffracted field is here meant either the field transmitted through the body or the field reflected thereby, so that the analysis of the diffraction may take place either during transmission, or reflection, or both at the same time.
Thus, by way of example, the defects of a wooden plank may be analyzed, the degree and homogeneity of drying of a panel or layer of plaster or of a sheet of paper checked, the development and result of the polymerization of a plastic material checked or the stage of de-freezing of foods evaluated.
A measuring device of this type is already known, described in U.S. Pat. No. 4,552,151. In this device, the doublet antennae are connected end to end so as to form the columns of a "matrix retina", i.e. the assembly of the doublet antennae form a rectangular panel. The collector means are formed by a stack of waveguides, each in the form of a horn and the assembly is controlled so that the value of the field may be measured successively at any point of the panel.
Such a two dimensional device is however fairly cumbersome for the length of each horn is of the order of size of the largest dimension of its openings, which substantially defines the maximum analysis length. This device is in addition of a relatively high cost price and it is relatively complex to use, particularly considering the necessity of using a microwave multiplexer. In addition, although it is suitable for biomedical applications, or laboratory applications, it is ill-fitted to industrial applications of the above type.